CHEMISTRY THE CENTRAL SCIENCE
22 CHEMISTRY OF THE NONMETALS
22.10 THE OTHER GROUP 4A ELEMENTS: Si, Ge, Sn, AND Pb
The trend from nonmetallic to metallic character as we go down a family is strikingly evident in group 4A. Carbon is a nonmetal; silicon and germanium are metalloids; tin and lead are metals. In this section we consider a few general characteristics of group 4A and then look more thoroughly at silicon.
General Characteristics of the Group 4A Elements
The group 4A elements possess the outer-shell electron configuration ns2np2. The electronegativities of the elements are generally low (TABLE 22.8); carbides that formally contain C4– ions are observed only in the case of a few compounds of carbon with very active metals. Formation of 4+ ions by electron loss is not observed for any of these elements; the ionization energies are too high. The +2 oxidation state is found in the chemistry of germanium, tin, and lead, however, and it is the principal oxidation state for lead. The vast majority of the compounds of the group 4A elements are covalently bonded. Carbon, except in highly unusual examples, forms a maximum of four bonds. The other members of the family are able to form more than four bonds. (Section 8.7)
TABLE 22.8 • Some Properties of the Group 4A Elements
Table 22.8 shows that the strength of a bond between two atoms of a given element decreases as we go down group 4A. Carbon-carbon bonds are quite strong. Carbon, therefore, has a striking ability to form compounds in which carbon atoms are bonded to one another in extended chains and rings, which accounts for the large number of organic compounds that exist. Other elements can form chains and rings, but these bonds are far less important in the chemistries of these other elements. The Si — Si bond strength (226 kJ/mol), for example, is much lower than the Si — O bond strength (386 kJ/mol). As a result, the chemistry of silicon is dominated by the formation of Si — O bonds, and Si — Si bonds play a minor role in silicon chemistry.
Occurrence and Preparation of Silicon
Silicon is the second most abundant element, after oxygen, in Earths crust. It occurs in SiO2 and in an enormous variety of silicate minerals. The element is obtained by the reduction of molten silicon dioxide with carbon at high temperature:
Elemental silicon has a diamond-type structure. Crystalline silicon is a gray metallic-looking solid that melts at 1410 °C. The element is a semiconductor, as we saw in Chapters 7 and 12, and is used to make solar cells and transistors for computer chips. To be used as a semiconductor, it must be extremely pure, possessing less than 10–7% (1 ppb) impurities. One method of purification is to treat the element with Cl2 to form SiCl4, a volatile liquid that is purified by fractional distillation and then converted back to elemental silicon by reduction with H2:
The process known as zone refining can further purify the element (FIGURE 22.33). As a heated coil is passed slowly along a silicon rod, a narrow band of the element is melted. As the molten section is swept slowly along the length of the rod, the impurities concentrate in this section, following it to the end of the rod. The purified top portion of the rod crystallizes as 99.999999999% pure silicon.
What limits the range of temperatures you can use for zone refining of silicon?
FIGURE 22.33 Zone-refining apparatus for production of ultrapure silicon.
Silicon dioxide and other compounds that contain silicon and oxygen make up over 90% of Earths crust. In silicates, a silicon atom is surrounded by four oxygens and silicon is found in its most common oxidation state, +4. The orthosilicate ion, SiO44–, is found in very few silicate minerals, but we can view it as the “building block” for many mineral structures. As FIGURE 22.34 shows, adjacent tetrahedra are linked by a common oxygen atom. Two tetrahedra joined in this way, called the disilicate ion, contain two Si atoms and seven O atoms. Silicon and oxygen are in the +4 and –2 oxidation states, respectively, in all silicates, so the overall charge of any silicate ion must be consistent with these oxidation states. Thus, the charge on Si2O7 is (2)(+4) + (7)(–2) = –6; it is the Si2O76– ion.
In most silicate minerals, silicate tetrahedra are linked together to form chains, sheets, or three-dimensional structures. We can connect two vertices of each tetrahedron to two other tetrahedra, for example, leading to an infinite chain with an …O—Si — O — Si … backbone. As Figure 22.34(b) shows, this chain can be viewed as repeating units of the Si2O64– ion or, in terms of its simplest formula, SiO32–. The mineral enstatite (MgSiO3) consists of rows of single-strand silicate chains with Mg2+ ions between the strands to balance charge.
In Figure 22.34(c) each silicate tetrahedron is linked to three others, forming an infinite sheet structure. The simplest formula of this sheet is Si2O52–. The mineral talc, also known as talcum powder, has the formula Mg3(Si2O5)2(OH)2 and is based on this sheet structure. The Mg2+ and OH– ions lie between the silicate sheets. The slippery feel of talcum powder is due to the silicate sheets sliding relative to one another.
FIGURE 22.34 Silicate chains and sheets.
Many minerals are based on silicates, and many are useful as clays, ceramics, and other materials. Some silicates, however, have harmful effects on human health, the best-known example being asbestos, a general term applied to a group of fibrous silicate minerals. The structure of these minerals is either chains of silicate tetrahedra or sheets formed into rolls. The result is that the minerals have a fibrous character (FIGURE 22.35). Asbestos minerals were once widely used as thermal insulation, especially in high-temperature applications, because of the great chemical stability of the silicate structure. In addition, the fibers can be woven into asbestos cloth, which was used for fireproof curtains and other applications. However, the fibrous structure of asbestos minerals poses a health risk because the fibers readily penetrate soft tissues, such as the lungs, where they can cause diseases, including cancer. The use of asbestos as a common building material has therefore been discontinued.
FIGURE 22.35 Serpentine asbestos.
When all four vertices of each SiO4 tetrahedron are linked to other tetrahedra, the structure extends in three dimensions. This linking of the tetrahedra forms quartz (SiO2). Because the structure is locked together in a three-dimensional array much like diamond (Section 12.7) quartz is harder than strand- or sheet-type silicates.
SAMPLE EXERCISE 22.9 Determining an Empirical Formula
The mineral chrysotile is a noncarcinogenic asbestos mineral that is based on the sheet structure shown in Figure 22.34(c). In addition to silicate tetrahedra, the mineral contains Mg2+ and OH– ions. Analysis of the mineral shows that there are 1.5 Mg atoms per Si atom. What is the empirical formula for chrysotile?
Analyze A mineral is described that has a sheet silicate structure with Mg2+ and OH– ions to balance charge and 1.5 Mg for each 1 Si. We are asked to write the empirical formula for the mineral.
Plan As shown in Figure 22.34(c), the silicate sheet structure is based on the Si2O52– ion. We first add Mg2+ to give the proper Mg : Si ratio. We then add OH- ions to obtain a neutral compound.
Solve The observation that the Mg : Si ratio equals 1.5 is consistent with three Mg2+ ions per Si2O52– ion. The addition of three Mg2+ ions would make Mg3 (Si2O5)4+. In order to achieve charge balance in the mineral, there must be four OH– ions per Si2O52– ion. Thus, the formula of chrysotile is Mg3(Si2O5)(OH)4. Since this is not reducible to a simpler formula, this is the empirical formula.
The cyclosilicate ion consists of three silicate tetrahedra linked together in a ring. The ion contains three Si atoms and nine O atoms. What is the overall charge on the ion?
Quartz melts at approximately 1600 °C, forming a tacky liquid. In the course of melting, many silicon–oxygen bonds are broken. When the liquid cools rapidly, silicon–oxygen bonds are re-formed before the atoms are able to arrange themselves in a regular fashion. An amorphous solid, known as quartz glass or silica glass, results. Many substances can be added to SiO2 to cause it to melt at a lower temperature. The common glass used in windows and bottles, known as soda-lime glass, contains CaO and Na2O in addition to SiO2 from sand. The CaO and Na2O are produced by heating two inexpensive chemicals, limestone (CaCO3) and soda ash (Na2CO3), which decompose at high temperatures:
Other substances can be added to soda-lime glass to produce color or to change the properties of the glass in various ways. The addition of CoO, for example, produces the deep blue color of “cobalt glass.” Replacing Na2O with K2O results in a harder glass that has a higher melting point. Replacing CaO with PbO results in a denser “lead crystal” glass with a higher refractive index. Lead crystal is used for decorative glassware; the higher refractive index gives this glass a particularly sparkling appearance. Addition of nonmetal oxides, such as B2O3 and P4O10, which form network structures related to the silicates, also changes the properties of the glass. Adding B2O3 creates a “borosilicate” glass with a higher melting point and a greater ability to withstand temperature changes. Such glasses, sold commercially under trade names such as Pyrex® and Kimax®, are used where resistance to thermal shock is important, such as in laboratory glassware or coffeemakers.
Silicones consist of O — Si — O chains in which the remaining bonding positions on each silicon are occupied by organic groups such as CH3:
Depending on chain length and degree of cross-linking, silicones can be either oils or rubber-like materials. Silicones are nontoxic and have good stability toward heat, light, oxygen, and water. They are used commercially in a wide variety of products, including lubricants, car polishes, sealants, and gaskets. They are also used for waterproofing fabrics. When applied to a fabric, the oxygen atoms form hydrogen bonds with the molecules on the surface of the fabric. The hydrophobic (water-repelling) organic groups of the silicone are then left pointing away from the surface as a barrier.
GIVE IT SOME THOUGHT
Distinguish among the substances silicon, silica, and silicone.